Kaposi's sarcoma-associated herpesvirus (KSHV), also called human herpesvirus-8 (HHV-8), is closely associated with the most common malignancies in AIDS patients, Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL). Increasing evidence suggest that lytic cycle of KSHV plays critical roles in development of KS and other KSHV-associated diseases. Therefore, switch of the virus from latency to lytic cycle infection is not only important for viral propagation, but also crucial for viral pathogenicity. The switch of KSHV between latency and lytic replication is controlled by a few regulatory proteins encoded by immediate-early (IE) genes of the virus. Our laboratory has been interested in the mechanism of KSHV reactivation and its roles in viral pathogenicity. Several IE genes were identified in the KSHV genome and some, including ORF50, K8 and ORF45, have been characterized in our laboratory during the past budget period. This proposal reflects our effort to continue the investigation on functional roles of KSHV immediate-early genes in viral life cycle and pathogenicity. In the next budget period, our research will be focused on the immediate- early protein ORF45. (1) ORF45 was found to inhibit activation of IRF-7 and virus-mediated interferon production. However, the mechanism underlying the ORF45 function remains elusive. Three hypotheses are proposed and will be examined. We will determine whether ORF45 blocks IRF-7 activation through (i) inhibiting its conformational change, or (ii) interfering with its phosphorylation in viral infected cells, or (iii) blocking its interaction with other cellular protein(s). (2) Our preliminary studies suggested that ORF45 may function in antagonizing IFN-associated antiviral responses and this immune evasion mechanism may be necessary for a successful viral infection and reactivation. We will determine roles of ORF45 in blocking host cell antiviral responses and facilitating KSHV infection and reactivation. In addition, the results from the proposed studies will provide assessment of whether targeting KSHV IE proteins ORF45 could be new strategies for therapeutic intervention for KSHV-associated diseases.